As the usage of optical components and systems has advanced there is an increasing desire to produce integrated optical systems. One component used in optical systems is an isolator, which permits transmission of light in only one direction. Optical isolators are useful, for example, to help avoid reflected light from returning back to a source (e.g., a laser). Optical isolators are typically bulky components, however, having sizes on the order of millimeters, making it difficult to include them within an integrated optical system.
One way of constructing an isolator is to place a Faraday rotator between two polarizers having differing axes of polarization (typically 45 degrees). The Faraday rotator produces gradual rotation of the polarization angle of an electromagnetic wave (e.g., light) propagating through the device. Light traveling in one direction passes through the first polarizer and is rotated into alignment with the second polarizer. The rotation is non-reciprocal, however, so that light traveling in the opposite direction is polarized by the second polarizer and rotated so that it is orthogonal to the first polarizer and thus does not pass through.
Magnetic materials having a high Verdet constant, such as cerium-substituted yttrium iron garnet (Ce:YIG) or bismuth-substituted YIG (Bi:YIG) can be used as the Faraday rotator, as such materials can provide a relative large amount of rotation (e.g., 45 degrees for several 100 micrometers of thickness) while retaining good optical transmission. Unfortunately, even these types of devices are larger than desired.
Devices have been constructed that form magnetic photonic crystals by stacking together very thin layers of alternating materials types (e.g., magnetic YIG and non-magnetic gadolinium gallium garnet (GGG)). The magnetic photonic crystal slows the light, providing a longer effective optical length, allowing a reduced length of material to provide the same amount of Faraday rotation. Growth and shaping of the YIG and GGG materials is not easily performed in situ, however, so fabrication of the photonic crystal structure can be difficult and not easily performed in an integrated optical system.